In a continuous production line that produces a galvanized steel sheet, a vertical looper is provided as a measure against variation in the speed of conveying the steel sheet and also as a measure to connect the steel sheets. The vertical looper is configured to store a predetermined amount of steel sheet.
As illustrated in FIG. 15, the vertical looper includes plural pairs of upper and lower rolls (upper looper rolls 33 and lower looper rolls 34). The steel sheet 30 runs while being wound alternately between the upper and lower rolls. The upper looper rolls 33 are arranged at predetermined intervals on a looper carriage 32, with which the upper looper rolls 33 are horizontally suspended. The looper carriage 32 is coupled, at four corners thereof, by four respective chains or wire ropes 36 via sprockets or sheaves 35 to drive sprockets or drums 38. Under control of a carriage driving mechanism 37, the drive sprockets or drums 38 take up or let out the chains or wire ropes 36 as necessary so that the upper looper rolls 33 are raised or lowered together with the looper carriage 32.
In a conventional vertical looper, due to tilting of a looper carriage caused by variation in the amount of elongation among chains or wire ropes during operation, or due to unevenness in the shape of a steel sheet, the steel sheet may snake and this may prevent proper operation. As a solution, the vertical looper has been operated under conditions where there is less occurrence of snaking. For example, the looper stroke (range of raising and lowering of the looper carriage) has been limited to reduce the occurrence of snaking. However, this has lowered the rate of capacity utilization. As another solution, snaking correction rolls may be installed in the vertical looper. However, the correction capability is limited because the number of installable rolls is limited due to space limitations.
Japanese Unexamined Patent Application Publication No. 8-267139 discloses a steel-sheet snaking preventing device for such a vertical looper. In that steel-sheet snaking preventing device for a vertical looper, as illustrated in FIG. 16, an edge position detector 42 detects a steel sheet edge on at least one of entry and exit sides of the vertical looper, and a displacement gauge 43 computes the amount of snaking of a steel sheet 30. On the basis of the result of this computation, a control means 44 drives jack mechanism driving sources 46 to cause jack mechanisms 41 to individually adjust the lengths of chains or wire ropes 36 that pull a looper carriage 32 so that the looper carriage 32 is tilted in the direction of the steel sheet width to prevent snaking of the steel sheet in the vertical looper.
The steel-sheet snaking preventing device for a vertical looper disclosed in Japanese Unexamined Patent Application Publication No. 8-267139 has a problem in that it makes the facility complex. That is, the chains or wire ropes may be elongated and the amount of elongation depends on the length and the load. Therefore, to tilt the looper carriage to prevent snaking on the basis of the detected amount of snaking of the steel sheet, the length of adjustment of each chain or wire rope needs to be determined by taking into account the overall length of the chains or wire ropes (i.e., looper carriage height) and the amount of elongation produced by load (tension) at that time. This requires detectors and controllers for the task, and thus makes the facility complex and costly.
Additionally, the conventional steel-sheet snaking preventing device for a vertical looper described above has a problem in that one-sided elongation of the steel sheet occurs when the looper stroke is short. A tensile stress σ applied to an end portion of the steel sheet by tilting the looper carriage can be expressed as σ=E·ε=E·(δ/L), where δ is an elongation at the end portion of the steel sheet, ε is an elongation strain, L is the steel sheet length between the upper and lower rolls, and E is the longitudinal elastic modulus of the steel sheet. If a value obtained by adding the unit tension UT of the steel sheet (which is obtained by dividing the steel sheet tension by the cross-sectional area of the steel sheet) to the tensile stress σ exceeds the yield point σy of the steel sheet (σy<σ+UT), one-sided elongation occurs due to plastic deformation of the end portion of the steel sheet. The smaller the steel sheet length L between the upper and lower rolls, the larger the tensile stress. Therefore, in a location where the looper stroke is short, it is necessary not only to tilt the looper carriage to prevent snaking, but also to tilt the looper carriage within a range where the one-sided elongation does not occur. However, one-sided elongation occurs in the conventional device because the looper carriage is tilted on the basis only of the amount of snaking of the steel sheet.
It could therefore be helpful to provide a steel-sheet snaking preventing device and a steel-sheet snaking preventing method for a vertical looper that can prevent snaking of a steel sheet with a simple facility.